1. Field of the Invention
The present invention relates to a switching power supply device in which a predetermined power supply voltage is generated at a secondary coil of a transformer by switching a current flowing through a primary coil of the transformer. More specifically, the present invention relates to a switching power supply device that is used as a power supply for optical disk devices such as a DVD recorder, a DVD player, or the like.
2. Description of the Related Art
Switching power supply devices have been used as power supplies for optical disk devices, such as DVD recorders, DVD players or the like. Generally, a switching power supply device according to the related art includes a transformer in which an induced voltage is output at a secondary coil through switching operation of a current flowing by a primary coil, an oscillation circuit that generates an oscillation signal for switching the current flowing through the primary coil of the transformer, an oscillation control circuit that controls the oscillation signal of the oscillation circuit on a basis of the voltage induced to the secondary coil of the transformer such that a DC voltage become have a predetermined value, a 12-volt generating circuit that generates a DC power supply voltage of a 12 V by the output voltage of the secondary coil of the transformer, a 5-volt generating circuit that generates a power supply voltage of 5 V by the output voltage of the secondary coil of the transformer, and a power supply feedback circuit that supplies an error signal to the oscillation control circuit as a feedback signal by comparing a reference voltage of 5 V generated on a basis of the power supply voltage of 5 V with the power supply voltage of 5 V and detecting a variation of the power supply voltage of 5 V as the error signal.
In addition, in another switching power supply device according to the related art, two transformers are used, and one of the two transformers is used for normal operation and the other is used for standby operation, so that the reduction of the power consumption in the standby state has been tried. However, since the two transformers are used, the power loss of the transformers is large, so that it is difficult to suppress the power consumption in the standby state to become a value equal to or smaller than 1 W.
FIG. 8 is a circuit diagram of a power supply feedback circuit included in a switching power supply device according to the related art. The power supply feedback circuit includes resistors R1 to R7 and R9, a capacitor C1, a transistor T1, a zener diode D1, and a photocoupler P. One end of the resistor R1 is supplied with a power supply voltage of 12V, and the other end of the resistor R1 is connected to a cathode of the zener diode D1 and one end of the resistor R2. An anode of the zener diode D1 is connected to a ground, and the other end of the resistor R2 is connected to one end of the resistor R3. The other end of the resistor R3 is connected to one end of the resistor R4 and one end of the resistor R5.
The other end of the resistor R4 is connected to a ground, and the other end of the resistor R5 is connected to one end of the resistor R6 and one end of the resistor R7. The other end of the resistor R6 is connected to one end of the capacitor C1, and the other end of the resistor R7 is connected to a base of the transistor T1. The other end of the capacitor C1 is connected to an emitter of the transistor T1 that is supplied with a power supply voltage of 5V. A collector of the transistor T1 is connected to an anode of a light-emitting diode D2 included in the photocoupler P through the resistor R9. The cathode of the light-emitting diode D2 is connected to a ground. The collector and the emitter of the phototransistor T2 included in the photocoupler P are connected to the above-mentioned oscillation control circuit.
A voltage of a connection point B between a reference voltage line and a control line is set to have a predetermined voltage of 5 V through the operation of the resistors R1 to R4 and the zener diode D1. The reference voltage line includes a connection point between the resistors R3 and R4, and the control line includes a base line of the transistor T1 which serves as an error detection element to detect an error signal generated when the power supply voltage of 5 V varies. The resistor R6 and the capacitor C1 serve to compensate for a phase at the time when the transistor T1 is operated.
Here, if the power supply voltage of 5 V supplied with the emitter of the transistor T1 increases to exceed the voltage of 5V, a collector current of the transistor T1 increases, so that the increased collector current is input to the photocoupler P through the resistor R9. Thereby, in the light-emitting diode D2 of the photocoupler P, a current increases, so that an amount of emitted light increases. As a result, in the phototransistor T2 which receives the increased amount of emitted light, an equivalent resistance is reduced. Accordingly, the oscillation control circuit, which will be described in detail below, controls an oscillation signal of the oscillation circuit and controls the power supply voltage having exceeded the voltage of 5 V so as to return to the original voltage of 5 V.
In contrast, if the power supply voltage of 5 V decreases to become smaller than the voltage of 5 V, the collector current of the transistor T1 decreases, so that the decreased collector current is input to the photocoupler P through the resistor R9. Thereby, in the light-emitting diode D2 of the photocoupler P, the current decreases, so that the amount of emitted light decreases. As a result, in the phototransistor T2 which receives the decreased amount of emitted light, the equivalent resistance is increased. Accordingly, the above-mentioned oscillation control circuit controls an oscillation signal of the oscillation circuit and controls the power supply voltage having become smaller than the voltage of 5 V so as to return to the original voltage of 5 V. As such, the power supply feedback circuit 9 performs feedback operation so as to maintain the power supply voltage of 5 V output from the above-mentioned 5-volt generating circuit as the predetermined voltage of 5 V.
However, in an optical disk device on which the switching power supply device according to the related art is mounted, in order to reduce the power consumption in a standby state, a power supply is cut off for a tuner or a fluorescent display tube in the standby state. However, it was difficult to maintain the power consumption in the standby state as a value equal to or smaller than 1 W. Accordingly, using a microcomputer included in the optical disk device, an intermittent oscillation pulse is generated from the microcomputer and is then supplied to a connection point B of a power supply feedback circuit shown in FIG. 8. In addition, the transistor T1 is switched, and the low power consumption is achieved. However, since the microcomputer is operated with a power supply voltage of 3.3 V smaller than the power supply voltage of 5 V, a current flows backward from the connection point B to the microcomputer, so that the microcomputer cannot be normally operated.
According to the related art disclosed in JP-A-2000-287444, in a switching power supply circuit which performs switching operation intermittently in a standby state, an intermittent frequency can be switched, and a stable power supply voltage corresponding to a secondary load of a transformer can be supplied. However, as described above, it is not possible to reduce the power consumption in the standby state using an intermittent oscillation clock output from a microcomputer operated with a voltage lower than a voltage of a main power supply output from the switching power supply device.
According to the related art disclosed in JP-A-60-16178, when a power supply is supplied to a load necessitating a low power, a switching element is turned on or turned off by an intermittent oscillation system, so that a power in accordance with an amount of a power of a power supply to be supplied is supplied. However, even in this technology, it is not possible to reduce the power consumption in the standby state using an intermittent oscillation clock output from a microcomputer operated with a voltage lower than a voltage of a main power supply output from the switching power supply device.